Composition and applications of a multi-component benzo[1,2-B:4,5-B] dithiophene-difluorothienothiophene randomly substituted polymers for organic solar cells

ABSTRACT

A polymer having two different sets of repeat units consisting essentially of: 
                         
In this polymer, R1, R2, R3 and R4 can be independently selected from the group consisting of alkyl group, alkoxy group, aryl groups and combinations thereof. Also the combination of R1, R2, R3 and R4 are not all identical and n and o are greater than 1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims thebenefit of and priority to U.S. Provisional Application Ser. No.62/005,046 filed May 30, 2014, entitled “Composition and Applications ofa Multi-Component Benzo[1,2-B:4-5-B] Dithiophene-DifluorothienothiopheneRandomly Substituted Polymers for Organic Solar Cells,” which is herebyincorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

FIELD OF THE INVENTION

This invention relates to compositions and applications for amulti-component benzo[1,2-B:4,5-B] dithiophene-difluorothienothiophenepolymer.

BACKGROUND OF THE INVENTION

Solar energy using photovoltaic effect requires active semiconductingmaterials to convert light into electricity. Currently, solar cellsbased on silicon are the dominating technology due to their highconversion efficiency. Recently, solar cells based on organic materialsshowed interesting features, especially on the potential of low cost inmaterials and processing. Judging from the recent success in organiclight emitting diodes based on a reverse effect of photovoltaic effect,organic solar cells are very promising.

Organic photovoltaic cells have many potential advantages when comparedto traditional silicon-based devices. Organic photovoltaic cells arelight weight, economical in the materials used, and can be deposited onlow cost substrates, such as flexible plastic foils. However, organicphotovoltaic devices typically have relatively low quantum yield (theratio of photons absorbed to carrier pairs generated, or electromagneticradiation to electricity conversion efficiency), being on the order of1% or less. This is, in part, thought to be due to the second ordernature of the intrinsic photoconductive process. That is, carriergeneration requires exciton generation, diffusion and ionization. Thediffusion length of an exciton is typically much less than the opticalabsorption length, requiring a trade off between using a thick, andtherefore resistive, cell with multiple or highly folded interfaces, ora thin cell with a low optical absorption efficiency.

Conjugated polymers are polymers containing π-electron conjugated unitsalong the main chain. They can be used as active layer materials forsome types of photo-electric devices, such as polymer light emittingdevices, polymer solar cells, polymer field effect transistors, etc. Aspolymer solar cell materials, conjugated polymers should possess someproperties, such as high mobility, good harvest of sunlight, goodprocessibility, and proper molecular energy level. Some conjugatedpolymers have proven to be good solar cell materials. Conjugatedpolymers are made of alternating single and double covalent bonds. Theconjugated polymers have a δ-bond backbone of intersecting sp² hybridorbitals. The p_(z) orbitals on the carbon atoms overlap withneighboring p_(z) orbitals to provide π-bonds. The electrons thatcomprise the π-bonds are delocalized over the whole molecule. Thesepolymers exhibit electronic properties similar to those seen ininorganic semiconductors. The semiconducting properties of thephotovoltaic polymers are derived from their delocalized π bonds.

There is a need in the art for polymer solar cells that exhibitincreased solar conversion efficiency.

BRIEF SUMMARY OF THE DISCLOSURE

A polymer having two different sets of repeat units consistingessentially of:

In this polymer, R1, R2, R3 and R4 can be independently selected fromthe group consisting of alkyl group, alkoxy group, aryl groups andcombinations thereof. Also the combination of R1, R2, R3 and R4 are notall identical and n and o are greater than 1.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and benefitsthereof may be acquired by referring to the follow description taken inconjunction with the accompanying drawings in which:

None

DETAILED DESCRIPTION

Turning now to the detailed description of the preferred arrangement orarrangements of the present invention, it should be understood that theinventive features and concepts may be manifested in other arrangementsand that the scope of the invention is not limited to the embodimentsdescribed or illustrated. The scope of the invention is intended only tobe limited by the scope of the claims that follow.

“Alkyl,” as used herein, refers to an aliphatic hydrocarbon chains. Inone embodiment the aliphatic hydrocarbon chains are of 1 to about 100carbon atoms, preferably 1 to 30 carbon atoms, more preferably, 1 to 20carbon atoms, and even more preferably, 1 to 10 carbon atoms andincludes straight and branched chains such as methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, n-pentyl, isopentyl,neo-pentyl, n-hexyl, and isohexyl. In this application alkyl groups caninclude the possibility of substituted and unsubstituted alkyl groups.

“Alkoxy,” as used herein, refers to the group R—O— where R is an alkylgroup of 1 to 100 carbon atoms. In this application alkoxy groups caninclude the possibility of substituted and unsubstituted alkoxy groups.

“Aryl” as used herein, refers to an optionally substituted, mono-, di-,tri-, or other multicyclic aromatic ring system having from about 5 toabout 50 carbon atoms (and all combinations and subcombinations ofranges and specific numbers of carbon atoms therein), with from about 6to about 10 carbons being preferred. Non-limiting examples include, forexample, phenyl, naphthyl, anthracenyl, and phenanthrenyl. Aryl groupscan be optionally substituted with one or with one or more Rx. In thisapplication aryl groups can include the possibility of substituted arylgroups, bridged aryl groups and fused aryl groups.

A polymer having two different sets of repeat units comprising:

In this embodiment R1, R2, R3 and R4 are independently selected from thegroup consisting of alkyl group, alkoxy group, aryl groups andcombinations thereof and the combination of R1, R2, R3 and R4 are notall identical. Additionally, in this embodiment n and o are greater than1.

In one embodiment the aryl groups comprise of heterocycles and fusedheterocycles.

In another embodiment the ratio of

in the polymer is around 50:50.

It is theorized that the introduction of the fluorine atom can lower thepolymer HOMO energy level and thus will lead to elevated open circuitvoltage in photovoltaic devices. The fluorine atom is known to inducebetter planar molecular conformation in poly-thiophene systems. Theformation of a difluorothienothiophene (DFTT) created similar resultswith better charge transport capability and higher short circuit currentand fill factor.

Typically, the number average molecular weight of the polymers is in therange of approximately 1000 to 1,000,000, with ideal polymers having anumber average molecular weight in the range of about 5000 to 500,000,and some ideal polymers having a number average molecular weight in therange of approximately 20,000 to 200,000. It will be appreciated thatmolecular weight can be varied to optimize polymer properties and theinventions of the present disclosure cover all molecular weights. Forexample, lower molecular weight can ensure solubility, while a highermolecular weight can ensure good film-forming properties.

In one embodiment the conjugated polymer can either be regio-regular orregio-random.

The polymers produced from the present disclosure can be used as activelayer material or photovoltaic materials in electronic devices orphotovoltaic devices such as photodetector devices, solar cell devices,and the like. Photovoltaic devices, including solar cell devices, aregenerally comprised of laminates of a suitable photovoltaic materialbetween a hole-collecting electrode layer and an electron-collectinglayer. Additional layers, elements or a substrate may or may not bepresent. In one embodiment the electronic devices are field effecttransistors, light emitting devices, and sensors, electrochromic devicesand capacitors.

As shown in the examples below the production of a polymer with twodifferent sets of repeat units appears to provide increased performance.

Examples:

List of Acronyms Used:

BDT: Benzo[1,2-b:4,5-b′]dithiophene

DFTT: 2,3 -difluorothieno[3,4-b]thiophene

FTT: 3 -Fluorothieno[3,4-b] thiophene

FTT(E): 2-ethylhexyl 3 -fluorothieno[3,4-b] thiophene-2-carboxylate

FTT (K1): 2-ethyl-1 -(3 -fluorothieno[3,4-b]thiophen-2-yl)hexan-1 -one

PCE: power conversion efficiency

Jsc: short circuit current

Voc: open circuit voltage

PDI: polydispersity index

M_(n): number average molecular weight defined by (ΣNiMi)/ΣNi where Miis the molecular weight of a chain and Ni is the number of chains ofthat molecular weight

Soxhlet Extraction: The polymer is washed using a reflux apparatus withdifferent solvents. The solvent and polymer is then heated till thesolvent evaporates into a gas, then cools into a liquid. The solvent isthen evaporated off and polymer products are produced.

EXAMPLES

Example 1=DFTT (45 mg, 0.135 mmol, 1.0 equiv), FTT(K1) (59.6 mg, 0.135mmol, 1.0 equiv) and BDT8 (275 mg, 0.270 mmol, 2.0 equiv) were dissolvedin a mixture of toluene (10 ml) and DMF(2 ml). The reaction solution wassparged with Ar for 20 minutes. Pd(PPh₃)₄ (11.0 mg, 4% mol) was added tothe reaction mixture, and then sparged with Ar for additional 10minutes. The solution was heated to 110° C. overnight. The dark bluesolution was precipitated into methanol (120 ml) and the solid wascollected by filtration. The solid was then dissolved in chloroform andallowed pass through a short column (silica gel). After concentration byroto-vap, the polymer solution was precipitated into hexanes. The solidwas collected by centrifugation and dried in vacuum. The product was adark blue solid (112 mg, 45.2%).

Example 2=DFTT (50 mg, 0.150 mmol) and BDT8 (152 mg, 0.150 mmol) weredissolved in a mixture of toluene (5 ml ) and DMF (1 ml). The reactionsolution was sparged with Ar for 20 minutes Pd(PPh₃)₄ (6.9 mg, 4% mol)was added to the reaction mixture, then sparged with Ar for an addition10 minutes. The solution was heated to 110° C. overnight. The dark bluesolution was precipitated into methanol (120 ml) and the solid wascollected by filtration. The solid was then dissolved in chlorobenzeneand allowed to pass through a short column (silica gel). Afterconcentration by roto-vap, the polymer solution was precipitated intohexanes. The solid was collected by centrifugation and dried in vacuum.The product was a dark blue solid (114 mg, 88.45).

Example 3=DFTT (30.3 mg, 0.091 mmol, 0.3 equiv), FTT-E (100.2 mg, 0.212mmol, 0.7 equiv) and BDT (274 mg, 0.30 mmol, 1.0 equiv) were dissolvedin a mixture of toluene (10 ml) and DMF (2 ml). The reaction solutionwas sparged with Ar for 20 minutes. Pd(PPh3)4 (14.0 mg, 4% mol) wasadded to the reaction mixture, then sparged with Ar for additional 10minutes. The solution was heated to 110° C. overnight. The dark bluesolution was precipitated into methanol (120 ml) and the solid wascollected by filtration. The solid was then dissolved in chloroform andallowed pass through a short column (silica gel). After concentration byroto-vap, the polymer solution was precipitated into hexanes. The solidwas collected by centrifugation and dried in vacuo. The product was adark blue solid (238 mg, 92.6%).

Device Fabrication and Measurement

Fabrication procedure of a regular device structure(ITO/PEDOT:PSS/active layer/PFN/Al): The polymer and PC₇₀BM weredissolved in o-xylene in a 1:1.6 (10 mg mL⁻¹: 26 mg mL⁻¹, respectively)weight ratios. The solution was stirred at 70° C. overnight, and thenfiltered using a 2.7 μm glass fiber filter. Prior to use, a 2.5% volumeratio of 1,8-diiodooctane (purchased from Sigma Aldrich) was added tothe solution. The solution was left to stir on the hotplate at 70° C.prior to use.

Indium tin oxide (ITO) patterned glass substrates were cleaned bysonication using the following solvents for each step: acetone,detergent water, deionized water, acetone, and isopropanol. Cleanedsubstrates were left to dry in the oven overnight.Poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) was thenspin-coated on ITO/glass substrates at 4000 rpm for 20 s and thenannealed at 150° C. for 10 min. The active layer solution was thenspin-coated on top of the PEDOT:PSS-coated ITO/glass substrates. Thecoated samples were left to dry under vacuum for 1 hr. A layer of PFNwas deposited prior to electrode deposition. Thermal evaporation wasused to deposit the electrode Aluminum (800Å). The samples wereencapsulated prior to removing them from the glove box for testing. Thedevices with active area of 0.041 cm₂ were tested using a 100 mW/cm₂ (AM1.5G) solar simulator.

Inverted structure fabrication: Indium tin oxide (ITO) patterned glasssubstrates were cleaned by sonication using the following solvents foreach step: acetone, detergent water, deionized water, acetone, andisopropanol. Cleaned substrates were left to dry in the oven overnight.ZnO solution (2.5 mg/ml in BuOH) was spin-coated on top of ITO, andthermally annealed at 120° C. for 5 min. The active layer solution wasthen spin-coated, and the devices were left to dry under vacuum for 1hr. Thermal evaporation was used to deposit the electrodes: MoO₃ 12 nmand Ag 160 nm. The samples were encapsulated prior to removing them fromthe glove box for testing. The devices with active area of 0.041 cm₂were tested using a 100 mW/cm₂ (AM 1.5G) solar simulator.

Table 1 depicts the solar cell performance of polymers from Examples1-3.

V_(oc) J_(sc) Fill Factor PCE Polymer Device Structure (V) (mA/cm²) (%)(%) Example ITO/PEDOT: 0.88 11.01 52.6 5.10 1 PSS/CS38: PC₇₀BM/PFN/AlExample ITO/PEDOT: 0.83 11.76 62.3 6.18 2 PSS/CS39: PC₇₀BM/PFN/AlExample ITO/PEDOT: 0.74 14.12 67.4 7.12 3 PSS/CS42: PC₇₀BM/PFN/Al

The process of producing a polymer with two different sets of repeatunits was combined with a novel DFTT co-monomer.

The present embodiment describes a process of dissolving3-fluoro-4,6-dihydrothieno [3,4-b]thiophene in a solvent to create asolution. An initiator is then added to the solution to produce aninitiated solution followed by adding a fluorinated chemical to theinitiated solution to produce2,3-difluoro-4,6-dihydrothieno[3,4-b]thiophene.2,3-difluoro-4,6-dihydrothieno[3,4-b]thiophene is then oxidized with anoxidant to produce 2,3-difluorothieno[3,4-b]thiophene. A brominatingstep then occurs to the 2,3-difluorothieno[3,4-b]thiophene to produce4,6-dibromo-2,3-difluorothieno[2,3-c]thiophene.4,6-dibromo-2,3-difluorothieno[2,3-c]thiophene is then debrominated andstille coupled to

In this embodiment the stoichiometric ratio of (f+g)≈h and f, g and hare not equal to 0. Additionally, in this emboidemnt R1, R2, R3 and R4are independently selected from the group consisting of alkyl group,alkoxy group, aryl groups and combinations thereof and where thecombination of R1, R2, R3 and R4 are not all identical.

In this embodiment the solvent used could be tetrahydrofuran, diethylether or hexanes.

In this embodiment the initiator used could be n-butyllithium

In this embodiment the fluorinated chemical could beN-fluorobenzenesulfonimide.

In this embodiment the oxidant could be meta-chloroperoxybenzoic acid

In this embodiment the bromination could occur with N-bromosuccinimide

Example 4=2,3-difluoro-4,6-dihydrothieno[3,4-b]thiophene

An oven-dried, 500 ml flask equipped with a magnetic stir bar wascharged with 3-fluoro-4,6-dihydrothieno[3,4-b]thiophene (2.2 g, 13.75mmol) and anhydrous THF (200 ml) under argon. The solution was cooled at−78° C. under a dry-ice acetone bath. N-butylithium (2.5 M, 6.0 ml, 1.1eq) was added dropwise. After cooling at −78° C. for half an hour, thereaction mixture was allowed to warm up to room temperature for 2 hours.The reaction vessel was recooled to −78° C. and (PhSO₂)₂NF (4.8 g in 80ml THF) was added dropwise. The mixture was warmed up to roomtemperature again for another 2 hours. After being quenched with 100 mlwater, the organic phase was separated. The aqueous phase was extractedusing CH₂Cl₂. The combined organic phase was dried over anhydrous sodiumsulfate. After removal of the solvent, the white solid product wasobtained by column chromatopgraphy (1.08 g, 44.1% GC-MS found m/q: 178;calculated for C₆H₄F₂S₂ 178.22)

Example 5=2,3-difluorothieno[3,4-b]thiophene

An oven-dried 250 ml flask equipped with a magnetic stir bar was chargedwith 560 mg of 2,3-difluoro-4,6-dihydrothieno[3,4-b]thiophene (3.15mmol) and 50 ml methylene chloride. The solution was cooled to 0° C.m-CPBA(0.71 g, 77%, 3.15 mmol) was added to one portion and the reactionmixture was allowed to warm up to room temperature overnight. Thesolvent was removed by a roto-vap and the residue was redissolved in 15ml acetic anhydride and heated for 2 hours. The acetic anhydride wasthen removed by roto-vap and the product was purified by columnchromatography to produce 199.5 mg of product. (GC-MS found m/q: 176(Calculated for C₆H₂F₂S₂ 176.21)

Example 6=4,6-dibromo-2,3-difluorothieno[2,3-c] thiophene: DFTT

2,3-difluoro-4,6-dihydrothieno[3,4-b]thiophene (195 mg, 1.11 mmol) wasdissolved in 10 ml for dry DMF. This solution was cooled at 0° C. In oneportion, 592 mg NBS (3.0 eq) was added and the reaction mixture wasstirred at 0° C. for 2 hours then warmed up to room temperatureovernight. The reaction mixture was then poured into 5% Na₂S₂O₃ aqueoussolution and extracted with methylene chloride. The organic layer wasdried over Na₂SO₄. After removal of the solvent, the residue waspurified by column chromatography to produce 198 mg (yield 53.5%) of awhite solid product. (GC-MS found m/q: 334 (Calculated for C₆Br₂F₂S₂334.7)

In closing, it should be noted that the discussion of any reference isnot an admission that it is prior art to the present invention,especially any reference that may have a publication date after thepriority date of this application. At the same time, each and everyclaim below is hereby incorporated into this detailed description orspecification as an additional embodiment of the present invention.

Although the systems and processes described herein have been describedin detail, it should be understood that various changes, substitutions,and alterations can be made without departing from the spirit and scopeof the invention as defined by the following claims. Those skilled inthe art may be able to study the preferred embodiments and identifyother ways to practice the invention that are not exactly as describedherein. It is the intent of the inventors that variations andequivalents of the invention are within the scope of the claims whilethe description, abstract and drawings are not to be used to limit thescope of the invention. The invention is specifically intended to be asbroad as the claims below and their equivalents.

The invention claimed is:
 1. A polymer having two different sets ofrepeat units consisting essentially of:

wherein R1, R2, R3 and R4 are independently selected from the groupconsisting of alkyl group, alkoxy group, aryl groups and combinationsthereof and where if R1=R2 then R2≠R4 and if R2=R4 then R1≠R2; and n ando are greater than
 1. 2. The polymer of claim 1, wherein the aryl groupscomprise of heterocycles and fused heterocycles.
 3. The polymer of claim1, wherein the conjugated polymer is regio-regular.
 4. The polymer ofclaim 1, wherein the conjugated polymer is regio-random.
 5. The polymerof claim 1, wherein the conjugated polymer is used as photovoltaicmaterial in one or more photovoltaic devices.
 6. The polymer of claim 5,wherein the one or more photovoltaic devices are polymer solar celldevices or photodetector devices.
 7. The polymer of claim 1, wherein theconjugated polymer is used as active layer material in one or moreelectronic devices.
 8. The polymer of claim 7, wherein the one or moreelectronic devices are field effect transistors, light emitting devices,and sensors, electrochromic devices and capacitors.
 9. The polymer ofclaim 1, wherein the ratio of

in the polymer is around 50:50.